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Bert Delisle

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Re: Stuart Batty On TenonsReply #2 - Nov 10th, 2013 at 10:47pm

Good video, that is the way I was taught to make tenons. I only have one set of jaws that are straight bore. I use Vicmarc and Oneway chucks, they both need to have tenons bevelled to hold properly. And Yes, I have had some pop out of the chuck, , learning opportunity.

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steve rost

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Re: Stuart Batty On TenonsReply #5 - Nov 12th, 2013 at 6:45am

Thanks Ron/Len. Everything I have learned has been from videos. I have added Stuart to my favorites. I watched vid after vid last nite. He makes it very clear! Something I need to go back and watch again is when he works on the inside of the bowl he cuts only with the tip and trailing edge. I have seen the tool rotated more and cutting with the tip and leading edge! Also his 40/40 grind on his bowl gouge. Good Stuff!

I just learned something. I had never heard of putting a chamfer on my tenons.

Same here. I'm still trying to figure out why it works since the break would occur above the chamfer.

It makes no sense to me either, the physics don't work.The only thing I can see it helping with is, eliminating a sharp edge that could catch on the jaw and potentially start a split as the blank is already coming out of the chuck.Just my two cents

"Right now I'm having amnesia and deja vu at the same time.I think I've forgotten this before." - Steven Wright

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robo_hippy

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Re: Stuart Batty On TenonsReply #8 - Nov 14th, 2013 at 6:20pm

The 'chamfer' is another word for dove tail. Stuart uses the Vicmark chucks, and the dove tail angle is about 7 degrees. I use them also. This chamfer only works with dove tail jaws, and the angle on the tenon or recess has to match the angle of the chuck jaws. A shoulder is added stability. When using a recess, you have the shoulder on the inside. He does really clamp down tight on them. Also, it looked like the one tenon he broke off was black locust. The more brittle woods do break off more easily than the more medium or softer woods, which will 'bend but not break' better.

No, it's not. They are almost oppositesLook at 12:03 into the video. He points his pencil at the chamfered (outside) edge of the tenon. This is the confusing part I am referring to.I fully understand the properties of using dovetail jaws, but I can see no real benefit to chamfering the edge of the tenon.

"Right now I'm having amnesia and deja vu at the same time.I think I've forgotten this before." - Steven Wright

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Len Layman

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Re: Stuart Batty On TenonsReply #12 - Nov 14th, 2013 at 9:11pm

If I remember right he also mentions that with the chamfer it may come out of the chuck but not break the tenon so it would be re-chucked. Perhaps the chamfer makes this possible. giving it a chance to "escape" as opposed to break.Any thoughts on that theory?

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robo_hippy

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Re: Stuart Batty On TenonsReply #15 - Nov 15th, 2013 at 12:18pm

Well, I may have not interpreted it correctly. He never mentions the word dove tail, and that is what he makes on the tenon to mount it into the Vicmark chucks. When I took the workshop with Stuart and his dad, 5 or so years ago, Stuart had me making tenons for some bowls we were turning. They were maybe 5 inches tall, and about the same diameter, so more of a closed form compared to what I normally turn. I made a tenon maybe 1/2 inch deep, and he had me round off the corners on the tenon for no more than 1/4 inch being in actual contact with the chuck jaws. He said it made the grip weaker. Not sure if I got an explanation or not. If the chuck jaw/tenon angles match perfectly, it should not be a problem. For sure if they are not a perfect match, the piece could rock as you are turning, and that would make it want to shear off. For my recesses that I use, I am seldom more than 1/8 inch deep, even for 16 inch diameter bowls. Of course, doing a lot of bowls and experimenting a lot helps me figure out what will or will not work.

I also think Len got it. I had a class with him years ago and checked my notes. I had a diagram of a foot with a chamfer and my notes said it provides additional mass. In other words technically the tenon only needed to be as long as the beginning of the chamfer. But the chamfer allows you to have a little more wood on the tenon - reducing the chance of the tenon splitting based on the grain direction or punky wood. Clearly he does not put a chamfer on the spindles since he shows there is no chance of the them splitting if there is a bad catch.

I was not putting a chamfer on my bowl blanks. I now have a better understanding because of this thread, and will start adding a chamfer - just in case.

I had a diagram of a foot with a chamfer and my notes said it provides additional mass. In other words technically the tenon only needed to be as long as the beginning of the chamfer. But the chamfer allows you to have a little more wood on the tenon - reducing the chance of the tenon splitting based on the grain direction or punky wood.

For those of you wanting to try this method, you need to allow yourself the extra room for cutting a chamfer.Example, if you usually cut a 1/8" tenon and now you want one with a chamfered edge, you need to cut a 1/4" tenon so that you will have enough area for your normal 1/8" tenon surface for the chuck to hold onto and an extra 1/8" area for the chamfer.

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Ed Weber

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Re: Stuart Batty On TenonsReply #19 - Nov 17th, 2013 at 11:03am

Quote:

There is nothing added in the grip line that would improve the strength of the tenon. Having more wood 1/4" away from the stress point will not improve the strength of the stress point.

Jim, while I agree with you, some think adding the "additional mass" that Mike mentioned is some kind of benefit. I was just try to help As I have mentioned twice already, I think he is simply trying to salvage the tenon from splitting off.He mentions and shows this at roughly 8:00 - 8:15 into the video

I know I'll most likely get grief for this but;If this whole issue is about making proper tenins for safety reasons, there are two things that SB does that I would not.#1. The tenons he cuts are too large for the jaws he is using.You should not be able to see 3/8"-1/2" distance between Jaws. #2, Over-tightening the Jaws. Anyone familiar with Vicmarc Jaws knows that they have a pretty sharp edge on them. The way he bears down on the chuck key when he's tightening, it's a wonder he doesn't shear off the tenon right there Make the tenon fit the jaws and there is no need to tighten that much.

I will agree that Stuart puts way more pressure on the tenon than I ever would. The plastic on the hex wrench will break off, and I have several that have done it. Putting too much pressure can put the wood under so much stress that it can split if you have any tiny catch. Just the 'straw that broke the camel's back' thing. It may add to 'more metal in contact with the wood', but still, that is too much for me.

As for the chamfer adding mass to the tenon, well, it may do that, but nothing is added to the contact points. I think that is like people turning out the inside of the bowl and leaving the center high because it adds mass. Well, it does, but not at the point where it will add anything.

Stuart does make some good points in his turning methods, but I don't agree with all of them.

I just learned something. I had never heard of putting a chamfer on my tenons.

Same here. I'm still trying to figure out why it works since the break would occur above the chamfer.

The chamfer changes the lever arm (shortens it) and the force is directed differently. The effect is to avoid starting the splitting along the end grain. The bowl will tear out of the jaws before it the grain splits.

If you have bad wood, cracks, etc, all the chamfering in the world cannot avoid breaking/splitting.

Hmm, I think most of my splits are on the side grain rather than on end grain. I do tend to orient my chuck jaws so they are not directly on the end grain, so not at 12/3/6/9 o'clock, but at 1:30/4:30/7:30/10:30. Just seems to hold a little bit better.

I still don't understand the use of a chamfer. I just make a shorter tenon. Same difference.

It's very clear why SB uses this method, but the interesting thing to me is why it works.It ONLY works if you have a tenon that is too large for the jaws.You can see in the video, (8:20) after he forces the blank out of the chuck, there are really only 8 points of contact. The center of each jaw did little, if any damage to the tenon at all. If the tenon was cut closer to the same diameter as the jaws this would not work and the chamfer would not help. The closer the tenon and jaws match, the better the mechanical connection is and the less pressure is need to apply for a secure grip.It does seem that he uses this method of a larger tenon intentionally for the purpose of re-chucking should a failure occur. I suppose some would call this planning ahead, I'm not sure.I would prefer to have a more secure grip initially, than what I would consider a less secure grip with the chance of re-chucking.JMHONo matter what method you use, please turn safe.

I just learned something. I had never heard of putting a chamfer on my tenons.

Same here. I'm still trying to figure out why it works since the break would occur above the chamfer.

The chamfer changes the lever arm (shortens it) and the force is directed differently. The effect is to avoid starting the splitting along the end grain. The bowl will tear out of the jaws before it the grain splits.

If you have bad wood, cracks, etc, all the chamfering in the world cannot avoid breaking/splitting.

I've seen bowls break off at the tenon where the tenon is still firmly mounted in the jaws. (maybe the jaws were clamped down to tight and the bowl snapped off when a catch occurred.)

As we all know, the break happens along the grain. There is a different force on the bowl when the lathe is spinning compared to when the lathe is still. When the lathe is spinning it is almost like a twisting/rocking motion instead of the force being applied to one single spot. Did you ever look the mark a gouge left after a catch? Its not centralized, instead its a few inches long covering 20%-60% of the circumference. The closer to the rim the more force is applied during a catch (think levers) and in a wider area as compared to a palm of a hand banging on a mounted bowl.

I think a better demonstration would be for Stuart to bang on the bowl then spin it an inch then bang on the bowl again (repeat if necessary).

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Ken Vaughan

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Re: Stuart Batty On TenonsReply #29 - Nov 18th, 2013 at 7:15pm

Let me try this another way ---

Good design and good work practice is to provide for the most safe failure. Think of break away light poles along high speed highways.

The tenon design is to fail safe. If the tenon is short, the champfer may not be needed to assure the failure will not likely be tenon separation.

Look at the champfer as a way to shorten an overly long tenon, and look more at the effective contact length of the tenon. Also note that the hinge point is at the outside contzct point of the outside of the jaws and the flat contact point on the bowl. Batty uses the chamfer to set the failure to the less risk mode of the tenon coming out of the chuck if an intense catch occurs

As is always the case, there are many schools of thought.It doesn't appear that in the video SB is targeting safety with this method, as much as not wasting wood. Whether the tenon breaks off in the jaws, or comes out of the chuck, it makes little difference, the blank is still flying free. So after changing your underwear, the only difference is that in SB's method, you may still be able to re-chuck your piece using the same tenon. Except now your chamfered tenon has damage from being pulled out of the chuck and most likely won't have as secure of a connection as it did originally.

As to Ron's point, I had similar thoughts. I think trying to leverage a blank out of the chuck would make a more sensible test than hitting it from the headstock side. Maybe try something like screwing in two lag bolts, one in the center and another two or three inches from center. Then lock the spindle, get a pry-bar and try to twist the blank out. This might better mimic the twisting and the forces of leverage when you get a catch.Whatever your methods, turn Safely

Look at the champfer as a way to shorten an overly long tenon, and look more at the effective contact length of the tenon. Also note that the hinge point is at the outside contzct point of the outside of the jaws and the flat contact point on the bowl.

It does seem as if Batty is reducing the effective length of the tenon, possibly to allow it to come out of the chuck more easily in order to save the tenon, and that the chamfer still allows more wood behind the tenon, perhaps to provide additional support for the dovetail helping it to remain more intact in case of the bowl coming out of the chuck. In the tenon that failed, the dovetail was the full length of the tenon, and thereby I would think provide more gripping power. In addition the tenon that failed had no supporting edge on the bowl, providing a more acute angle of moment at the leverage point, contributing to the forces that caused the tenon to fail. These two factors would seem to provide a higher failure rate at the tenon, than the shorter effective length of dovetail of the chamfered tenon and it's supporting flat surface on the bowl that changes the angle at which the forces are applied on the tenon that did not fail.

Batty uses the chamfer to set the failure to the less risk mode of the tenon coming out of the chuck if an intense catch occurs

I not sure you can make that point. The flat surface that provides the contact point must surely help the stability of the blank, as pointed out by Batty when mounting the bowl without a contact surface on the bowl. The shorter effective tenon length of the chamfered tenon, would seem to me to allow the bowl to come off the chuck easier than a longer tenon, but help in leaving an intact tenon.

As I interpreted Batty, he is recommending (1) that the bowl have a flat surface for the face of the chuck jaws to provide stability and thus far less prone to stress the tenon, and (2) that the chamfered tenon provides a stronger attachment. I'm not sure that it provides a "stronger" attachment, but perhaps one that is less susceptible to damage, and is sufficiently strong enough to provide the means by which the bowl can be worked. He has certainly had far more experience than I will have accumulated for the rest of my turning years, and the points would seem to me in my very limited turning experience to have validity.

Good design and good work practice is to provide for the most safe failure.

Safety is always #1. The safest way to prevent a bowl from coming out of a chuck would be to use a tailstock. Like Ed pointed out, Stuart's technique is more about saving the blank than it is about safety.

Its been my experience that tenons will shear at the top of the jaws.

There have been times when I have a 1" long tenon with a shoulder 1/8" to 1/4" from the end. If the the blank shears off at the shoulder there is still enough tenon left to cut a new shoulder. It wouldn't be hard to reverse the piece and cut a new shoulder.

Take a look at the timestamp 8:47, notice there is a shoulder. Now look at timestamp 9:01, Stuart points out that there is no shoulder and mentions the problems working without a shoulder, good advice there. Shoulders are very important on a tenon and its were the blank should sit on.

Take a look at timestamp 9:36, Stuart is cranking down the jaws to a point where he might be helping the tenon to fail by making the jaws to tight, add a a minor catch (or in Stuart's case a few raps) and the tenon breaks off.

As I interpreted Batty, he is recommending (1) that the bowl have a flat surface for the face of the chuck jaws to provide stability and thus far less prone to stress the tenon, and (2) that the chamfered tenon provides a stronger attachment. I'm not sure that it provides a "stronger" attachment, but perhaps one that is less susceptible to damage,